1. Field of the Invention
The invention relates to a slurry hydrocarbon synthesis process, which includes increasing the activity of fresh catalyst in-situ in the slurry liquid, during the synthesis reaction. More particularly the invention relates to a slurry Fischer-Tropsch hydrocarbon synthesis process for producing hydrocarbons from a synthesis gas, wherein the activity of fresh synthesis catalyst added to the slurry reactor is increased in-situ in the slurry during the synthesis reaction.
2. Background of the Invention
The slurry Fischer-Tropsch hydrocarbon synthesis process is now well known and documented, both in patents and in the technical literature. This process comprises passing a synthesis gas which comprises a mixture of H.sub.2 and CO, up into a reactive slurry comprising a hot, synthesized hydrocarbon liquid, in which a suitable, particulate Fischer-Tropsch type of catalyst is dispersed and suspended, at reaction conditions effective for the H.sub.2 and CO to react and form liquid hydrocarbons. The hydrocarbon liquid is continuously withdrawn from the reactor and upgraded by one or more upgrading steps, which include fractionation and conversion operations such as hydroconversion, in which a portion of the molecular structure of at least some of the hydrocarbon molecules is changed. The upgraded products may include, for example, a syncrude, various fuels and lubricating oil fractions and wax. During the synthesis reaction, the presence of reversibly deactivating catalyst species present in the synthesis gas, such as NH.sub.3 and HCN, causes a reduction in catalyst activity. This activity loss is reversible and may be restored in-situ in the reactor by rejuvenation, as is known and disclosed, for example, in U.S. Pat. No. 5,283,216; 5,811,363; 5,811,468; 5,817,702; 5,821,270; 5,844,005 and 5,958,986. Catalyst rejuvenation is achieved by passing hydrogen or a hydrogen treat gas into the bottom of a vertically oriented, hollow conduit or tube immersed in the slurry or external of the synthesis reactor in a separate vessel. This sets up a slurry circulation in the tube, in which the hydrogen gas contacts the catalyst particles in the slurry liquid, thereby rejuvenating them by restoring at least part of their activity. Some of the activity loss is permanent, which ultimately requires catalyst regeneration or replacement. One of the advantages of the slurry process, is the ability to add and remove catalyst without taking the reactor off-line and thereby stopping the synthesis reaction. U.S. Pat. No. 5,292,705 discloses that the activity of a fresh Fischer-Tropsch type of hydrocarbon synthesis catalyst can be increased by treatment with hydrogen in a hydrocarbon liquid, including in-situ in the hydrocarbon synthesis slurry in the synthesis reactor. The hydrogen treatment for increasing the catalytic activity is preferably conducted in the absence of CO. Thus, in this process, if the catalyst activity is increased by treatment in-situ in the hydrocarbon synthesis slurry in the slurry synthesis reactor, the synthesis reactor is not producing hydrocarbons while the activity of the fresh catalyst is increased. It would be an improvement to the art if the activity of fresh catalyst added to the reactor could be increased while the reactor is producing hydrocarbons and without adversely effecting the selectivity or productivity of the synthesis reaction.